1. Field of the Invention
This invention relates to gas turbine and, more particularly, to an arrangement for reducing thermal stresses in turbine rotors.
2. Description of the Prior Art
Present day gas turbines includes rotors characteristically comprising a central disc or hub having a peripheral rim and a plurality of blades extending radially outwardly from the rim. In general, the rotors are cast or forged as an integral unit and the blades are relatively small and thin in comparison with the central disc. Because of this construction, a steep temperature gradient exists at the junction of the blades and the rim during turbine start-up because the relatively small thin blades reach an equilibrium temperature much faster than the rim. This steep temperature gradient produces local thermal stresses which can lead to a reduction in the useful life of the rotor. Another factor that adversely affects rotor life is the maximum allowable turbine inlet temperature, with higher, more efficient temperature shortening the life of the rotor.
In order to reduce the effects of thermal stresses and therefore extend the useful life of such rotors, the rotors are designed with ultra-conservative, allowable stress limits. Such designs require relatively heavy rotors, and the use of such heavy rotors increases manufacturing costs and can also effect turbine operational efficiency.
A number of patents deal with problems relating to turbine temperatures and employ heating and cooling techniques in attempting to solve these problems. For example, U.S. Pat. No. 966,588 (Parsons) discloses a technique of heating turbine parts wherein the turbine spindle is heated to maintain predetermined clearances. U.S. Pat. No. 3,903,691 (Szydlowski) discloses a technique for avoiding the formation of thermal imbalance in turbine engines wherein compressed air is injected into the engine in proximity to the rotating parts as soon as the engine stops.
Further, U.S. Pat. No. 2,919,103 (Schelp) discloses an arrangement for cooling the peripheral rim of a centripetal turbine wheel during operation so as to reduce the temperature range between operation and shut down. Also U.S. Pat. No. 3,286,461 (Johnson) discloses a turbine engine which is started by applying compressed air against the blade. Cooling is also provided during operation. U.S. Pat. No. 3,588,265 (Berry) discloses a computer used in maintaining predetermined temperatures in a turbine. However, in summary these devices are not completely satisfactory for reducing thermal stresses in the rotor.